Addition versus radiolytic production effects of hydrogen peroxide on aqueous corrosion of UO₂

The effects of hydrogen peroxide, H₂O₂, on UO ₂ corrosion is investigated in aerated deionized water in two types of situations. The H₂O₂ species is either added to water or produced by radiolysis at UO₂/H₂O interfaces. The concentrations vary in the range 10^-5-10^-1 mol l ^-1. The radiolysis is induced by irradiating the UO ₂/H₂O interfaces with a He²⁺-beam emerging from the UO₂ discs into the solutions. Both the evolution of the aqueous solutions and the UO₂ surfaces are characterised. In both types of experiments, the alteration of UO₂ results in the formation of the same secondary phase, an hydrated uranium peroxide called studtite (UO₂(O)₂ • 4H₂O). However, the uranium release at the interface differs strikingly. It is much higher when H ₂O₂ is produced by irradiation than when it is simply added. Furthermore, it varies in opposite direction as a function of the H ₂O₂ concentration. This gives evidence that the chemistry at the UO₂ interface under irradiation differs significantly from the chemistry induced by simply adding H₂O₂ to the solution. Rutherford backscattering spectrometry is used to determine the growth rate of the corrosion layer. For H₂O₂ addition, the layer thickness increases with increasing leaching time, although as time increases, the U release tends towards zero. It is possible to establish the first empirical equation relating the corrosion rates to the added H ₂O₂ concentrations. For H₂O₂ radiolytic production, the growth is continuous as irradiation time increases but the growth rate seems to decrease as the layer grows and to reach a limit.